On the dual thing - the only chance it has of working with any player is that if they are managed as dual players - so their training is tailored towards a guy that needs to maximise his abilities at both. It absolutely means that he won’t be at good at either as he would be if he concentrated on one code alone, but some players are good enough to make the first 15 in both codes and still be 10% off their best. If you have a guy that is the best for his position, should he really not be considered because he isn’t as good as he could be. The Christie at minor thing is mentioned above as an example of managers insisting on single code stuff - but that went horribly wrong.
At senior county level, I think it could be done, but it would probably have to be managed completely differently to how it was ever done before. The player would have to be trained as a dual player and not as a footballer one night and a hurler the next, the whole thing would have to be considered in one package. I assume then managers would be thinking why put themselves to the extra hassle. It would also have to be done from minor onwards or guys would drift away from one code or the other. O Callaghan is only still hurling at the level he is because of Cuala getting an extended run. If his hurling was only one or two championship games a year, he would have slipped back a lot by now. Not alone is it the skill level, you can actually work on that yourself a lot, but it is also how to challenge and how to win ball. If you are away from a game for an extended period you lose that ability to tackle and everything seems faster until you get back to speed. If you see Connolly hurling, he shows great flashes of skill, but rarely if ever completes a tackle in it - yet this is one of his core strengths at football.